The present invention relates to a control method and a control apparatus of a screw fastening apparatus, and more particularly, to a screw fastening apparatus whose reaction force is reduced so that the screw fastening apparatus can be held by one hand.
Conventionally, a power-assisted screw fastening apparatus is used for fastening a bolt or a screw with a predetermined torque. A screw fastening apparatus is generally controlled such that its shaft is continuously rotated to fasten a screw and when the torque reaches a certain value, the power is cut off or a clutch is slid.
Meanwhile, an operator holds the screw fastening apparatus with his/her hand for fastening a screw with respect to a work on a conveyer in various assembly lines in many cases.
In such a case, it is desired that the screw fastening apparatus can be held with one hand in view of operability. In the case of a one-handed type screw fastening apparatus, reaction of screw fastening operation must be received by one hand. Therefore, as the fastening torque is increased, there is a problem that its reaction becomes a load acting on the operator.
In the case of the above-mentioned apparatus whose shaft rotates continuously, since the reaction force of the fastening torque is received by the operator""s hand. directly, the load acting on the operator is great. In order to reduce the reaction force, an impact type apparatus utilizing impact by rotor inertia of a rotor is used.
In the conventional impact type screw fastening apparatus, however, since a screw is fastened through a collision energy generating mechanism and a socket, precision of the fastening torque is largely varied depending upon variation of transmitting efficiency of these members, and this apparatus is not suitable for fastening a screw with high precision. In order to enhance the precision, there is used a method in which a clutch mechanism is provided on a tip end of a shaft, and the clutch is slid to control the torque when excessive torque is inputted.
Under such circumstances, a screw fastening apparatus called oil pulse wrench having both of two functions (impact generating section and clutch mechanism) becomes widespread to solve the two problems, i.e., of reducing the reaction force and of enhancing the precision.
The oil pulse wrench usually includes an oil pulse section in which an air motor which is a driving source and a bypass valve which generates impulse-like oil pressure so as to use the generated oil pressure as a clutch mechanism are integrally constituted together.
However, the oil pulse wrench has the following structural problems.
(1) In order to control the fastening torque, it is necessary to adjust a hydraulic pressure of the bypass valve functioning as the clutch mechanism, but the hydraulic pressure must be adjusted whenever the target torque is changed.
(2) The oil pulse generating section is a mechanism for generating the impulse by compressing and decompressing the oil while rotating. Therefore, oil and constituent parts such as a compression blade are deteriorated and worn as being used. Thus, it is necessary to frequently readjust and replace the parts.
(3) Since the characteristics of the oil pulse wrench are varied depending upon an oil temperature also, there is a possibility that the precision of the fastening torque and ability of fastening step is varied.
(4) A heat radiation of the oil pulse section is great due to repetition of compression and decompression of oil. Therefore, the pulse generating mechanism is cooled utilizing emission of an air motor. For this reason, if the pulse generating mechanism is not cooled, its temperature is increased up to approximately 100xc2x0 C. for some minutes"" operation. Thus, there is a possibility that problems occur with respect to safety and stability of the function.
(5) For the above reasons, in the case of the oil pulse wrench, the necessity of using an air motor as a driving source is high and flexibility in selection of the driving source is low. Incidentally, an air motor has low energy efficiency as compared with an electric motor, and has a problem of dust and mist.
The present invention has been accomplished in view of the above problems, and it is an object of the invention to provide a screw fastening apparatus as well as a control method and a control apparatus of the screw fastening apparatus in which the various drawbacks of the conventional oil pulse wrench are solved by using an electric motor, the reaction force is small, and the precision is good.
A method according to the present invention is a control method of a screw fastening apparatus using an electric motor as a rotation driving source in which an output torque of the motor is generated like pulses, as well as an actual torque is detected at every predetermined time interval, the output torque is controlled such that the output torque is maintained when a detected torque value does not exceed a maximum value of the detected torque values detected heretofore, and such that the output torque is increased by a predetermined amount when the detected torque value exceeds the maximum value of the detected torque values detected heretofore, and the motor is stopped when the detected torque value reaches a target value.
Further, current pulse is intermittently supplied to the motor, an actual torque is detected at every predetermined time interval, a current value is maintained when a detected torque value does not exceed a maximum value of the detected torque values detected heretofore, and the current value is increased by a predetermined amount when the detected torque value exceeds the maximum value of the detected torque values detected heretofore and the supply of the current pulse is stopped when the detected torque value reaches a target value.
Preferably, the current pulse is such that ON-time and/or OFF-time thereof can be variably set.
Size of the detected torque value is judged and an increase of the current value is calculated irrespective of ON and OFF of the current pulse.
The motor is controlled in speed and allowed to rotate at high speed until the screw sits, and after the screw seats, the control as described above is carried out.
Further, pulse-like current that is gradually increased is supplied to the motor, thereby generating gradually increasing torque like pulses in the motor, and the motor is stopped when a maximum value of the torque reaches a target value.
An apparatus according to the present invention includes torque detecting means for detecting a fastening torque of the screw by the motor, setting means for setting a target value of the fastening torque, a current command calculating section which intermittently supplies current pulse to the motor, which calculates at predetermined time intervals to maintain a current value when a detected torque value does not exceed a maximum value of the detected torque values detected heretofore and which calculates to increase the current value by a predetermined amount when the detected torque value exceeds the maximum value of the detected torque values detected heretofore and a stop control section for stopping the supply of the current pulse when the detected torque value reaches the target value.